One of the deadliest hurricanes in recent memory and the most destructive in US history, Hurricane Katrina profoundly affected New Orleans and its surroundings, where water reached up to 20 kilometres (12 miles) from the shore. Hurricane Katrina was the child of a waning tropical depression and an atmospheric trough known as a tropical wave. It moved across the Gulf of Mexico and rapidly strengthened over unseasonably warm waters, transforming into a maximum-rated category 5 hurricane and shifting away from Florida shortly before it slammed into the vulnerable city of New Orleans in south-east Louisiana.